1. Field of the Invention
The present invention relates to a rotating electrical machine for a vehicle including a field winding, and particularly to noise reduction of a generator or a generator motor.
2. Description of the Related Art
A magnetic field generated by a rotor magnetic pole of an AC generator or an AC generator motor mounted in a vehicle is not a complete sine wave, but has a waveform including higher harmonics. Besides, a magnetic field by a generating current flowing through an armature winding also includes higher harmonics, and by the interaction of both magnetic fluxes including the higher harmonics, magnetic sucking force is generated between a magnetic pole of a field core as a rotor and an armature core as a stator, and harmonic components of the sucking force generate electromagnetic oscillation, and generate noise, so-called electromagnetic noise. Besides, the harmonic components of the magnetic flux generate torque ripple when a generator motor is made to function as a motor, and generate output ripple when it is made to function as a generator, and degrades the characteristics.
In a conventional rotating electrical machine for a vehicle, as shown in FIG. 14 or FIG. 17 of JP-A-2001-218395, a magnetic pole of a rotor is shaped substantially into a trapezoid when viewed from a gap plane between the rotor and a stator, the opposite plane of the magnetic pole is also inclined so that its thickness becomes gradually thick from the tip to the root, and consideration is given so that harmonic components of the magnetic field generated by the rotor are lessened by using the effect of the skew. However, the manufacture of such a shape is not easy, and in order to simplify the shape, it is desirable that the inclined portion is eliminated as much as possible, and a linear shape parallel to a rotating shaft is formed.
In response to such a request, for example, a technique as described in JP-A-2001-218395 is disclosed. The technique disclosed in this document is such that a magnetic pole of a rotor is formed into a stair shape having plural parts in an axial direction, and the respective parts are set to have different widths in a rotation direction, so that pitches in the rotation direction between center lines of gaps of adjacent magnetic poles are changed stepwise in the axial direction, and the respective pitches in the rotation direction are set in a specified range.
In the magnetic pole formed into the stair shape as stated above, since there is a tendency that higher harmonics generated by the rotor become larger than those in the case of the substantially trapezoidal magnetic pole, it becomes necessary to reduce the higher harmonics. Thus, in the above document, it is assumed that the composition of the fifth and seventh harmonics generated by the rotor is the third harmonic, and the pitches in the rotation direction between the center lines of the gaps between the magnetic poles changing stepwise in the axial direction is set. However, the originally generated third harmonic can not be reduced by the measure relative to the composite value obtained on the assumption that the fifth and seventh harmonics have the wave height values of 1/n, and it is necessary to consider a direct reduction measure against the third harmonic having the highest wave height value.
Besides, in the magnetic pole formed into the stair shape, as compared with the conventional magnetic pole formed into substantially the trapezoid, since there is a tendency that the mass at the tip of the magnetic pole becomes large, the stress exerted on the root of the magnetic pole becomes large by the centrifugal force, and at the time of high speed rotation, the possibility that the mechanical reliability is damaged, for example, the magnetic pole part is deformed also becomes large.